The imaging of the teeth and jaw area employs the so-called limited-area computer tomography devices, an example of which are cone-beam computed tomography imaging devices (CBCT). The device transilluminates the patient's skull by a beam in order to collect volume element data at a desired point in the skull and a three-dimensional image is reconstructed of the skull point in question based on the data produced. The devices are used in the teeth and jaw area specialty in diagnosing and planning treatment e.g. in procedures related to surgery, implantology and oral pathology.
An X-ray apparatus employed in limited-area computer tomography imaging typically comprises a rotating arm attached at one end of a supporting frame, at opposite ends of which are attached a device generating X-rays and a device detecting X-rays. Exposure to X-rays is performed by rotating the rotating arm, whereby the movements of the rotating arm and thus also the device generating X-rays and the device detecting X-rays are synchronised such that an image of an area of desired size e.g. of the patient's tooth or jaw joint is provided to the device detecting X-rays. Typically, the rotating arm is rotated around its stationary rotation axis. The device detecting X-rays is e.g. a CCD (charged coupled device) or a CMOS sensor which registers radiation having penetrated the object being imaged.
The diameter of the area being imaged is typically only a part of the diameter of the whole skull or jaw area. In X-ray imaging, it is important to position the imaging area of the object to be imaged precisely in a correct location in relation to the imaging apparatus. For imaging, it is also important that the object remains stationary. The positioning of the imaging area—and simultaneously the positioning of the patient to be imaged—is started by choosing and locating the object to be imaged, which can be assisted by using e.g. pointers and indicators implemented by lights or e.g. another X-ray image taken of the object.
For instance, from specification U.S. Pat. No. 6,118,842 is known an X-ray apparatus applicable for limited-area computer tomography imaging, where between the device generating X-rays and the device detecting X-rays is located an object positioning means for positioning the object to be imaged in the imaging area. In relation to their locations, the object positioning means, the device generating X-rays and the device detecting X-rays are relatively adjustable in the anteroposterior, lateral and vertical directions. From specification U.S. Pat. No. 6,118,842 is also known an X-ray apparatus for limited-area computer tomography imaging, where the location of the object positioning means positioned between the device generating X-rays and the device detecting X-rays is adjustable in relation to the frame of the X-ray apparatus in the anteroposterior, lateral and vertical directions. Then, the imaging area is located on a straight line according to the rotation axis of the rotating arm, around which the device generating X-rays and the device detecting X-rays rotate. The precise location of the object positioning means in said directions can be adjusted by an adjustment mechanism which is installed between the frame of the X-ray apparatus and the object positioning means. When adjusting the location of the object positioning means, it is possible to utilise object positioning data which has been obtained from a tomographic image taken earlier of the object by panoramic imaging. The adjustment of the location of the object positioning means can also utilise a light beam sent by a light indicator located on the rotation axis of the rotating arm, whereby the imaging area of the object is precisely in the correct place when the light beam focuses on the object positioning means or the object being imaged.
In the above limited-area computer tomography devices according to prior art, the object to be imaged is chosen one by one and the object to be imaged is positioned in the imaging area by the object positioning means one by one before performing the actual X-ray imaging. Then, a new object to be imaged is chosen and it is positioned in the imaging area by the object positioning means, after which the actual X-ray imaging is performed. The positioning of the imaging area is thus performed as its own separate measure for each object chosen to be imaged. Each object being imaged is imaged to imaging-specific coordinates determined by the positioning of the imaging area.
When wishing to use the prior-art apparatus to image e.g. the patient's both jaw joints, the first jaw joint to be imaged is first positioned by the object positioning means in its place in the imaging area, after which it is imaged, and then the second jaw joint to be imaged is positioned by the object positioning means in its place in the imaging area, after which it is imaged. In order to be able to image both jaw joints, the patient being imaged has to be positioned twice by the object positioning means to the imaging area, because both jaw joints are imaged separately. Each separate positioning of the patient for imaging takes time and consumes human and device resources. Furthermore, when the patient being imaged is positioned twice by the object positioning means to the imaging area for imaging the jaw joints, both jaw joints are imaged in their own coordinates. After this as an additional step, both images have to be registered in one way or another in the same coordinates before being able to make comparisons and measurements between the objects having been imaged. When the separately shot images are registered in the same coordinates, extra auxiliary means are required, the dimensions and geometry of which are known and which are visible in both images, or the images must have a common area which is visible in both images.
A problem of the above arrangements is also that they do not in any way consider the special characteristics related to the location or the resemblances of the object to be imaged. The human body has e.g. several imagable objects which are located in pairs on both sides of the body. Often, the imaging need relates to both such objects in order to be able to compare possible differences in the objects. Such objects in the human body are e.g. the above-mentioned jaw joints which are symmetrically located on two sides of the skull. When imaging by prior-art devices, both jaw joints must be positioned in the imaging area and imaged separately, whereby two positioning periods for positioning the imaging area are required to image the jaw joints. The two separate positionings of the imaging area also cause the fact that the jaw joints are imaged in different coordinates, the registering of which in the same coordinates requires the above-mentioned additional measures. Thus, imaging particularly objects located symmetrically on different sides of the body or the skull is slow and awkward.